Method of measuring the moisture content of a running web



p 1952 N. N. STRATVEIT ET AL. 2,611,974

METHOD OF MEASURING THE MOISTURE CONTENT OF A RUNNING WEB Filed se g. 26, 1949 2 SHEETS-SHEET 1 Flg.l.

P 1952- N. N. STRIKTVEIT ETAL 2,611,974

METHOD OF MEASURING THE MOISTURE CONTENT OF A RUNNING WEB Filed Sept. 26; 1949 Fig.5.

Patented Sept. 30, 1952 METHOD OF MEASURING THE MOISTURE CONTENT OF A RUNNING WEB Nils Nilsen Stratveit, 0510, and Arne Jfilifis some, Drammen, Norway Application September 26, 1949, Serial No. 117,805 In Norway August 11, 1945 17 Claims. 1

The present invention relates to a method of measuring the moisture content of a running web being dried by passage through a drying apparatus.

The invention further comprises a method of maintaining at a constant level the moisture content of such a web.

One object of the invention is to provide a method which can be carried out in a simple manner and by means of standard instruments.

Another object is to provide a method giving correct results even by varying thickness and/or velocity of the web.

A still further object is to provide a method which can be used in connection with different types of drying apparatuses.

The method according-to the invention can be used in connection with an instrument indicating the moisture content of the web, or in connection with a recording instrument.

Further the method can be used for automatic regulation of the moisture content of the web.

The web, the moisture content of which is to be measured or regulated, might be a paper web in a paper machine, a, cellulose web, a textile web or any other water containing web being dried in a drying apparatus.

The drying apparatus might comprise a number of heated cylinders in contact with which the web is being conveyed.

The drying apparatus might, however, instead of cylinders comprise heat radiating elements, the web being conveyed in proximity of the radiating surfaces without contacting the same.

Further the web might be dried by passage through a drying chamber through which hot air is conveyed.

A number of methods of measuring the moisture content of a running web have previously been proposed. The method according to the invention is distinguished from all these known methods thereby that the invention uses the decrease of the temperature of the web, caused by evaporation of water from the web as a measure of the moisture content.

The invention is based on the following recognition:

When a moist web is heated to a temperature above the temperature of the surroundings and is conveyed away from the heating source, the temperature of the web decreases.

This decrease is caused:

(a) By convection, (b) By radiation, and By evaporation of the moisture in the web.

When the temperature of the web is not too far below 100 C., for instance C., the part of the decrease caused by the evaporation is relatively great. Experiments have shown that when the moisture content is relatively high, the temperature decrease measured between two fixed points will be rather independent of the moisture content. When the moisture content is reduced below a certain limit however, the temperature decrease caused by the evaporation rapidly diminishes.

This fact is diagrammatically illustrated on the drawing, Figure 1. The curved line in this figure shows the relationship between the temperature decrease of a running web measured at two stationary points, and the moisture content of the web. As will be seen, the temperature decrease is between A and B of about the same size by varying moisture contents, and then diminishes rapidly from B to C.

The curved line on Figure 1, representing the functional relation between the temperature decrease and the moisture content of the web will of course be dependent of the nature of the web, the drying apparatus and the arrangement of the measuring points. In order to obtain a line as shown, the temperature of the first measuring point should be not too far below C., and the points of measurements should be so positioned that no substantial amount of heat is supplied to the web between the said points.

According to the present invention use is made of the above mentioned fact by using the decrease of the temperature of the web measured at two stationary points as a measure of the moisture content. Preferably the measuring points should be so positioned that the temperature decrease will correspond to a point of the curved line on Figure 1 between B and C.

The applicants have found that in order to secure satisfactory results, the measuring points should be arranged at a place where the moisture content is not more than 20%, preferably about 10%, whereas the temperature of the web at the first measuring point should preferably be at least 90 C. This limit is however, not critical.

Further the best results are obtained when the temperature of the web at the first measuring point does not vary considerably.

Further, the measuring points should be arranged so that no substantial amount of heat is supplied to the web between the said points.

The temperature decrease of the web between two points positioned as indicated above, is substantially caused by evaporation of water, and as this evaporation diminishes rapidly when the water content approaches zero, the temperature decrease will vary highly with varying moisture content as shown in Figure l.

The applicants have found that the temperature decrease measured in accordance with the invention is substantially independent of the thickness of the web. A variation of the thickness of the web of about will thus not cause any remarkable variation in the temperature decrease measured. This might be due to the fact that the web is porous and that the evaporation takes place not only from the plane surface of the web, but also from the surface of the pores, and that the eiiicient evaporation surface thus increases with increasing thickness of the web. This explanation might be correct or not; the fact remains that the method according to the invention gives results being substantially independent of the thickness of the web.

Further the applicants have found that the temperature decrease measured in accordance with the invention is substantially independent of the velocity of the web.

The temperature decrease might preferably be measured by means of one or more thermocouples, but the invention is not limited to the use of such instruments.

A number of thermocouples might be positioned with intervals in transverse direction of the web, in order to control a possible variation of the moisture content in this direction.

When using a thermocouple, the junctions might be positioned in contact with the web itself, or in contact with parts of the drying apparatus having approximately the same temperature as the web at a certain point. When the web, for instance, is dried by means of drying cylinders, the web during its contact with a cylinder will assume about the same temperature as the cylinder.

One junction of the thermocouple might therefore be placed in contact with a cylinder and the second junction in contact with the Web at some distance from the cylinder in the travelling direction of the web.

The electric current generated by the thermocouple might be used for controlling an instrument indicating the moisture content of the web, or for controlling an instrument recording the moisture content, or the current might be used for controlling devices by means of which the moisture content of the web is maintained at a constant level.

It should be pointed out, that it is previously known to obtain an electric current or an impulse being more or less proportional to the moisture content of arunning web being dried by passage through a drying apparatus, and using this current or impulse for automatic regulation of the moisture content of the web. The present invention does not consist in such an automatic regulation per se, but in the manner in which the moisturecontent is measured, or in other words, in using the temperature decrease of the web, substantially caused by partly evaporation of the moisture content of the web as a measure of the moisture content of the web.

In order that the invention might be more easily understood, it shall in the following be described in connection with the accompanying drawing, Figures 2-6, which by way of examples illustrate some embodiments of the invention.

As noted above, Figure 1 is a graph showing web temperature plotted against moisture content.

On the drawing Figure 2 shows a diagrammatical view of a drying apparatus comprising a number of drying cylinders in contact with which is being passed a moist web to be dried and which apparatus is provided with means for maintaining the moisture content of the web at a constant level in accordance with the method according to the invention.

Figure 3 shows a corresponding detail view of the same drying apparatus being provided with means for measuring the moisture content of the web in accordance with the method according to the invention.

Figure 4 is a View corresponding to Figure 3 and illustrates another embodiment of the invention.

Figure 5 is a diagrammatical view of an apparatus for drying a running web by means of heat radiating surfaces and provided with means for measuring the moisture content of the web in accordance with the invention.

Figure 6 is a corresponding view showing a modified arrangement of the measuring points.

Figure 7 shows a perspective view of an arrangement similar to Figure 3, comprising a num ber of thermocouples positioned with itnervals in transverse direction of the web.

In the following description with reference to the drawing, it is assumed that a thermocouple is used for measuring of the temperature decrease. However, it will easily be understood that the invention is not limited to the use of a thermocouple, and that any suitable instruments might be used for this purpose.

In Figures 3-6 the symbol designated L is supposed to be an instrument indicating the moisture content of the web. It will, however, be obvious to any person skilled in the art, that this symbol might as well indicate an instrument for recording of the moisture content or for regulation of the same.

Whereas the invention in the following is described in connection with the drawing, making the above mentioned suppositions, this is only made in order to make the invention easily understood, and the drawing, therefore, should not be regarded as any limitation of the scope of the patent.

- The drying apparatus illustrated on Figure 2 is supposed to comprise 13 drying cylinders of which only the first, C1, the eleventh, 011, and the twelfth, 012, are shown. D is a main steam supply pipe being common for all 13 cylinders. From the main pipe steam is supplied to the cylinders through branch pipes Eli-1a. Fl-lI; are regulation valves for individual regulation of the steam supply to the separate cylinders. G is a paper web to be dried by being passed through the apparatus in contact with the cylinders. The web enters the apparatus in moist condition at the left end of the drying apparatus, and leaves the apparatus in dry condition at its right end.

According to the invention the moisture content of the web at the dry end is measured or maintained at a constant level in the following manner. 4

A thermocouple TITZ is placed with its one junction T1 in contact with the web at the point where the web leaves the cylinder C11 and its other junction in contact with the web before it reaches the following cylinder C12.

The first measuring point T1 might however also be positioned a short distance away from the cylinder.

Instead of placing the measuring point-T1 in contact with the web, it might be placed in contact with the cylinder 011 for instance as shown in Figure 3, as the web during its contact with the cylinder assumes about the same temperature as the cylinder.

H is an amplifier by means of which the electric current generated by the thermocouple is amplified before it is led to the servomotor I controlling a valve K regulating the steam supply to the main steam pipe D.

The temperature of the web at T1 will ordinarily be about 100 C. The moisture content of the web at this place in the drying apparatus will in most cases be about When the web leaves the cylinder 011, the temperature of the web will rapidly decrease, so that the temperature of the web at T2 will be remarkably lower than at T1. This temperature decrease willas previously explained, vary highly with varying moisture content and be substantially independent of the velocity and thickness of the web, so that this temperature decrease with very good results might be used as a measure of the moisture content or for regulation purposes.

In Figure 3 the thermocouple is connected to a measuring instrument L. As already mentioned, the measuring point T1 is here placed on the surface of the cylinder C11.

Figure 4 shows the measuring of the decrease of the temperature of the web after it has left the last cylinder C13 of the drying apparatus. Ti-is here shown in contact with the last 0371- inder and T2 in contact with the web at some distance away from the last cylinder. The first measuring point might of course be placed on the web as shown on Figure 2.

Figure 5 shows diagrammatically the right end of a drying apparatus comprising a casing I in which are arranged a number of heat radiating elements 2 between which is left a zigzag passage for the web G. The web is passed over guide rollers 3, which are not heated. 4 is a discharge opening through which the web is led out from the casing I. The first measuring point T1 is here placed immediately within the discharge opening, whereas the second measuring point T2 is placed a distance away from the opening on the outside of the casing. The temperature decrease measured between T1 and T2 will give a very good indication of the moisture content of the web. The first measuring point might however also be positioned in the discharge opening or a short distance away from the same on the outside of the casing.

Figure 6 shows a modification of the arrangement shown on Figure '5. According to Figure 6 the first measuring point T1 is placed in contact with the heat radiating element 2a near the point at which the web is led away from this element. The temperature of the web at this point will approximately be the same as the temperature of the heating element.

Figure '7 shows diagrammatically the use of four thermocouples T1T2, T1T2, Ti'Tz' and T1"T2" arranged with intervals in transverse direction of the web.

By means of a rotary switch not shown, these thermocouples might alternatively be connected to an indicating or recording instrument. One of the thermocouples might be used for automatic regulation of the moisture content of the web. Other coupling arrangements are also possible. In all examples previously described a number of thermocouples as shown in Figure '7 might be used instead of a simple one.

'We claim:

1. The method of measuring the moisture content of a running web being dried by passage through a drying apparatus comprising the step of measuring the decrease of the temperature of the web caused by evaporation of part of the water content therefrom between two spaced fixed points at a place where the moisture content of the web is below 20%, said points being so located that no substantial amount of heat is supplied to the web between said two points.

2. The method of measuring the moisture content of a running web being dried by passage through a drying apparatus comprising the step of measuring the decrease of the temperature of the web caused by evaporation of part of the water content therefrom between two spaced fixed points at a place where the moisture content of the web is below 20% and the temperature of the web at the first measuring point is at least C., said points being so located that no substantial amount of heat is supplied to the web between said two points.

3. The method of measuring the moisture content of a running web being dried by passage through a drying apparatus comprising a series of drying cylinders in contact with which the web is being conveyed comprising the step of measuring the decrease of the temperature of the web caused by evaporation of part of the water content therefrom between the point where the web is leaving one of the drying cylinders at a place where the moisture content of the web is below 20% and the temperature of the web at a point spaced therefrom in the travelling direction of the web.

4. The method of measuring the moisture content of a running web being dried by passage through a drying apparatus comprising a series of drying cylinders in contact with which the web is being conveyed comprising the step of measuring the decrease of the temperature of the web caused by evaporation of part of the water content therefrom between the point where the web is leaving one of the drying cylinders of the drying apparatus at a place where the moisture content of the web is below 20% and a point spaced therefrom, the last mentioned point being so located that no substantial amount of heat is supplied to the web between the said points.

5. The method of measuring the moisture content of a running web being dried by passage through a drying apparatus comprising a series of drying cylinders in contact with which the web is being conveyed, comprising the step of measuring the difference caused by evaporation of part of the water content therefrom between the temperature of one of the drying cylinders at a place where the moisture content of the web is below 20% and the temperature of the web at a point spaced from said cylinder in the travelling direction of the web.

6. The method of measuring the moisture content of a running web being dried by passage.

through a drying apparatus comprising a series.

of drying cylinders in contact with which the web is being conveyed, comprising the step of measuring the decrease of the temperature of the web caused by evaporation of part of the water' content therefrom between the point where theweb is leaving the last drying cylinder and a point spaced therefrom in the travelling direc-- tion of the web.

7. The method of measuring the moisture con-- tent of a running web being dried by passage:

of drying cylinders in contact with which the web is being conveyed, comprising the step of measuring the diiference caused by evaporation of part of the water content therefrom between the temperature of the last drying cylinder and the temperature of the web at a point spaced from said cylinder in the travelling direction of the web.

8. The method of measuring the moisture content of a running web being dried by passage through a drying apparatus comprising the steps .of measuring the decrease of the temperature of the web caused by evaporation of part of the water content therefrom between two fixed points one of said points being located in the drying apparatus immediately before the web leaves the dryingapparatus and the other measuring point being located outside the drying apparatus a short distance away from the point where the 5,

web leaves the same.

9. The method of measuring the moisture content of a running web being dried by passage through a drying apparatus comprising a series of heat radiating elements having plane heat radiating surfaces in parallel with which the web is being conveyed comprising the step of measuring the difierence caused by evaporation of part of the water content therefrom between the temperature of the last heat radiating element and the temperature of the web at a point spaced from said element in the travelling direction of the web.

10. The method of maintaining at a constant value the moisture content of a running web being dried by passage through a drying apparatus comprising the step of measuring the decrease of the temperature of the web caused by evaporation of part of the water content therefrom between two fixed points at a place where the moisture of the web is below said points being so located that no substantial amount of heat is supplied to the web between said two points, and using this temperature decrease as an impulse for the regulation of the heat supply of the drying apparatus.

11. The method of maintaining at a constant value the moisture content of a running web being dried by passage through a drying apparatus comprising the step of measuring the decrease of the temperature of the web caused by evaporation of part of the water content therefrom between two spaced fixed points at a place where the moisture content of the web is below 20% and the temperature of the web at the first measuring point is at least 90 (1., said points being so located that no substantial amount of heat is supplied to the web between said two points, and using this temperature decrease as an impulse for the regulation of the heat supply of the drying apparatus.

12. The method of maintaining at a constant value the moisture content of a running web being dried by passage through a drying apparatus comprising the steps of measuring the decrease of the temperature of the web caused by evaporation of part of the water content therefrom between two fixed points one of said points being located in the drying apparatus immedi ately before the web leaves the drying apparatus and the other measuring point'being located out- 8 side the drying apparatus a short distance away from the point where the web leaves the same and using this temperature difference as an impulse for the regulation of the heat supply of the drying apparatus.

13. The method as claimed in claim 5 characterized therein that the temperature of the cylinder after which the temperature decrease is measured, is kept substantially constant.

14. The method as claimed in claim 8 characterized therein that the temperature of the web at the point where the web leaves the drying apparatus is kept substantially constant.

15. The method of maintaining at a constant value the moisture content of a running web being dried by passage through a drying apparatus comprising a series of drying cylinders in contact with which the web is being conveyed, comprising the steps of measuring the difference caused by evaporation of part of the water content therefrom between the temperature of the web close to the point where the web is leaving one of the drying cylinders near the dry end of the drying apparatus at a place where the temperature of the web is between -105 C. and the temperature of the web close to the point where the web arrives at the succeeding drying cylinder, and using this temperature difference as an impulse for the regulation of the heat supply of the drying apparatus.

16. The method of measuring the moisture content of a running web being dried by passage through a drying apparatus comprising a series; of drying cylinders in contact with which the web is being conveyed comprising the steps of measuring the differences caused by evaporation of part of the water content therefrom between the temperature of the web close to the point where the web is leaving one of the drying cylinders of the drying apparatus at a place where the temperature of the web is between 95- C.- and the temperature of the web close to the point where the web arrives at the succeeding drying cylinder.

17. The method of measuring the moisture content of a running web being dried by passage through a drying apparatus comprising a series of drying cylinders in contact with which the web is being conveyed comprising the steps of measuring the differences caused by evaporation of part of the water content therefrom between the temperature of the web close to the point where the web is leaving one of the drying cylintiers of the drying apparatus at a place where the temperature ofthe web is between 95-105 C. and the temperature of the web at a point spaced therefrom. a

' NILS N'ILSEN STRATVEIT.

ARNE JI'JLIfJs SCHIE.

REFERENUES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

